This invention relates to an optical element useful, for example, in the field of optoelectronics and photonics, such as optical communications and optical information processing, and to an optical control method and method of manufacturing an optical element using such an optical element.
In the field of optoelectronics and photonics, much research is being carried out on light and optical control methods which attempt to modulate the intensity (amplitude) or frequency (wavelength) of light without using electronic circuitry, by using the change of transmissivity or refractive index caused by irradiating an optical element produced by fabricating an optical material or optical composition with light.
When the properties of light are used to perform parallel optical logic computing and image processing, xe2x80x9cspacial light modulatorsxe2x80x9d for performing certain types of modulation such as varying the optical intensity distribution of the beam cross-section are of great importance, and optical control techniques are also expected to find application in this area.
Phenomena to which light and optical control methods are expected to be applied are saturable absorption, nonlinear refraction, nonlinear optical effects such as the photorefractive effect and photochromic phenomena, and these are now attracting wide attention.
A phenomenon is known wherein light in a first wavelength region excites absorption of light in a second wavelength region different from the first wavelength region without an accompanying change of molecular structure. This phenomenon may be referred to as xe2x80x9cexcited state absorptionxe2x80x9d, xe2x80x9cinduced absorptionxe2x80x9d or xe2x80x9ctransient absorption.xe2x80x9d
In one example of an attempt to produce excited state absorption, in Japanese Patent Laid-open publication No. Sho 53-137884, an optical conversion technique is reported wherein a liquid or solid containing a porphyrin compound and an electron acceptor is irradiated with at least two kinds of light of differing wavelength, and the information contained in light of one wavelength is transferred to light of the other wavelength. In Japanese Patent Laid-open publication No. Sho 55-100503 and Japanese Patent Laid-open publication No. Sho 55-108603, a propagation light is selected corresponding to the time-dependent variation of excited light, using the difference in the spectrophotometer spectrum between the ground state and excited state of organic compounds such as porphyrin derivatives.
In Japanese Patent Laid-open publication No. Sho 61-129621, a radiant energy transmission control method is disclosed comprising a step wherein a first photon flux is introduced into a barium crown glass fiber doped with uranium oxide without attenuation, the first photon flux is attenuated by introducing a second photon flux, energy level 2 of the fiber is populated, part of the first photon flux is absorbed to populate energy level 3 and part of energy level 3 then returns to energy level 2 so as to further attenuate the first photon flux.
Japanese Patent Laid-open publication No. Sho 63-89805 discloses a plastic optical fiber that contains an organic compound such as a porphyrin dielectric in its core having an absorption that corresponds to a transition to a triplet state higher than the triplet state excited by light. Japanese Patent Laid-open publication No. Sho 63-236013 discloses an optical functional device which performs switching wherein crystals of a cyanine dye such as cryptocyanine are irradiated with light of a first wavelength to excite the molecules, the molecules are irradiated with light of a second wavelength different from that of the first wavelength, and light of the second wavelength is either transmitted or reflected according to the optical excitation state due to light of the first wavelength. In Japanese Patent Laid-open publication No. Sho 64-73326, a light signal modulating medium is disclosed comprising a photo-induced electron shift material such as a porphyrin dielectric in a matrix material which is irradiated by light of a first and second wavelength, and optical modulation is performed using the difference of absorption spectrum between the excited and ground states of the molecule.
As optical apparatus used in this prior art, Japanese Patent Laid-open publication No. Sho 55-100503, Japanese Patent Laid-open publication No. Sho 55-108603 and Japanese Patent Laid-open publication No. Sho 63-89805 disclose a device structure wherein a light propagating optical fiber is wrapped around an exciting light source (for example, a flash lamp). In Japanese Patent Laid-open publication No. Sho 53-137884 and Japanese Patent Laid-open publication No. Sho 64-73326, a device is disclosed wherein the whole of a propagation part corresponding to signal light in a light-responsive optical element is irradiated with a control light which is not converged, and is in fact diverged by a means such as a projecting lens.
Other methods in the prior art perform modulation of light using the refractive index distribution due to a thermal effect. In the aforesaid prior art, studies are also being carried out on methods to modulate light using the refractive index distribution due to the thermal effect.
In Japanese Patent Laid-open publication No. Sho 59-68723, an optical modulator is disclosed wherein an electrical signal passes through a heat emitting resistor, and the wavefront of a light flux is modified due to a refractive index distribution in a liquid medium in which a refractive index distribution is produced by heat from the heat emitting means. It is described that a cycle is performed of the order of kHz or msec from formation to extinction of the refractive index distribution. Further, in Japanese Patent Laid-open publication No. Sho 60-130723, a method is disclosed for converting near infra-red control light into heat energy in a heat absorbing layer, transmitting this heat to a thermal effect medium via a near infrared light reflecting layer and visible light reflecting layer, and converting the wavefront of a light flux incident on the visible light reflecting layer using the refractive index distribution produced in the thermal effect medium.
However, in these methods for modulating light using refractive index distribution due to the aforesaid thermal effect, there is a long heat propagation path until a thermal effect is produced, and as the heat is propagated while the surface area of an increased temperature part increases relative to the surface area of the control beam, the volume and heat capacity of the propagation path increases, the usage efficiency of energy supplied from the control beam is low, and a high speed response cannot be expected.
In all of these prior art techniques, very high-density optical power is required to cause a change of transmissivity or refractive index that is sufficient for practical purposes. The response to optical irradiation is slow, fine adjustment of the optical system is necessary, and there is a large variation in the control light output if there is a slight change in the optical system. For these reasons, a practical system has not yet been developed.
To resolve the above problems in the prior art, the following two inventions have been disclosed relating to optical control methods and optical control devices which aim to induce an optical response of sufficient magnitude and speed from a photoresponsive optical element using as low a power as possible. Japanese Patent Laid-open publication No. Hei 8-286220 discloses an optical control technique wherein control light is made incident on an optical element comprising a photoresponsive composition, and intensity modulation and/or light flux density modulation of a signal light which passes through the optical element is performed by reversibly varying the transmissivity and/or refractive index of the signal light in a wavelength region different from that of the control light. The control light and signal light are respectively converged and irradiated to the optical element, and the optical paths of the control light and signal light are arranged so that the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light, overlap. In Japanese Patent Laid-open publication No. Hei 8-151133 and Japanese Patent Laid-open publication No. Hei 8-286220, a method is disclosed wherein, in a diverging signal light flux which has been transmitted through or reflected by an optical element, part of the flux in a region strongly affected by intensity modulation and/or light flux density modulation is separately extracted. Another method is disclosed wherein, in a diverging signal light flux which has been transmitted through or reflected by an optical element, part of the flux in a region strongly affected by intensity modulation and/or light flux density modulation is separately extracted by performing the extraction within an angular range (aperture angle) smaller than the divergence angle of the flux. Although these are very good methods, it is not easy to adjust an optical system to satisfy the necessary condition that xe2x80x9cthe control light and signal light are respectively converged and irradiated to an optical element, and the optical paths of the control light and signal light are arranged so that the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light, overlap,xe2x80x9d and the result is easily affected by changes in the component elements of the device.
It is therefore an object of this invention to resolve the above problems, and to provide an optical control method and device that provides a sufficiently strong and rapid optical response from a photoresponsive optical element using as low an optical power as possible. It is a further object of this invention to provide an optical control method and device that allow easy adjustment of an optical system, and allow some tolerance therein.
As an example of a method of manufacturing a plastic microlens, a method of manufacturing a refractive index distribution planolens from an organic polymer material (plastic) by osmosis and diffusion of a monomer is disclosed in xe2x80x9cM. Oikawa, K. Iga, T. Sanada: Jpn. J. Appl. Phys., 20(1), L51-L54 (1981)xe2x80x9d. In this method, a refractive index distribution lens is monolithically formed on a flat substrate by a monomer exchange technique. For example, methyl methacrylate (n=1.494) as a low refractive index plastic is diffused into a flat plastic substrate of polydiacrylisophthalate (n=1.570) which has a high refractive index from a 3.6 mm xcfx86 circular disk mask.
However, to manufacture a flat microlens having a predetermined refractive index distribution by this method, the setting of manufacturing conditions is a complex matter, such as the selecting of resin compositions having different refractive indices but which can be processed by the monomer exchange method, selecting the size of the above circular disk, and selecting the right temperature for monomer exchange.
A method of manufacturing a plastic microlens array by embossing the sheet surface of a thermoplastic polymer compound is described in P. Pantelis, D. J. Mccartney: Pure Appl.Opt., 3(2), 103-108 (1994). It is reported that a lens array comprising plural lenses of diameter approximately 1 to 2 mm may be manufactured by a high temperature technique applied to, for example, a polycarbonate sheet. In the case of this method, there is much room for improvement regarding the manufacture of the original plate to be embossed.
Further, in Y. Koike, A. Kanemitsu, Y. Shioda, E. Nihei, Y. Otsuka: Appl. Opt., 33(16), 3394-3400 (1994), it is reported that a refractive index distribution type polymer ball lens (xcfx86 0.5-1.1 mm) having a linear or second order spherical refractive index distribution and a small spherical aberration was manufactured by suspension polymerization of an acrylic resin. As can be easily understood, according to this method, there are many limitations in manufacturing a microlens according to design specifications by simultaneously controlling size and refractive index distribution.
This invention was conceived to overcome the above defects in the prior art, provide an optical element comprising a plastic microlens in which the size, shape and refractive index are controlled, and to provide a method of manufacturing the same.
To achieve the above objects, an optical element according to a first invention of this application comprises at least a light absorption film used to perform intensity modulation and/or light flux density modulation utilizing a thermal lens effect based on a reversible refractive index distribution produced by respectively converging a control light and a signal light of different wavelengths and irradiating them to the light absorption film, the wavelength of the control light being selected from the absorption band of the light absorption film, and bringing at least the control light to a focus in the light absorption film so as to produce a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region, the thickness of the light absorption film not exceeding twice the confocal length of the converged control light.
Herein, the signal light and control light are incident effectively perpendicular to the optical element to minimize losses due to reflection.
The confocal length mentioned here is the distance of an interval over which the light flux converged by a converging means such as a convex lens may be considered substantially parallel in the vicinity of the beam waist (focus). When the amplitude distribution of the electric field of the advancing beam cross-section, i.e., the energy distribution of the light flux, is a Gaussian beam having Gaussian distribution, a confocal length Zc is given by equation (1) using the circular constant xcfx80, a beam waist radius xcfx890 and wavelength xcex.
Zc=xcfx80xcfx8902/xcexxe2x80x83xe2x80x83(1)
Regarding the lower limit of the thickness of the light absorption film, it is preferable that this is as thin as possible provided that an optical response can be detected.
To achieve the above object, in an optical element according to a second invention of this application as defined in Claim 1, wherein a light transmitting heat insulation film is provided in the wavelength band of the control light and the signal light on either or both sides of the light absorption film.
To achieve the above object, in an optical element according to a third invention of this application as defined in Claim 1 or 2, wherein a heat transfer film is provided on either or both sides of the light absorption film when the heat insulation film is not present, and a heat transfer film is provided on either or both sides of the light absorption film through the intermediary of the neat insulation film when the heat insulation film is present.
To achieve the above object, in an optical element according to a fourth invention of this application as defined in any of Claims 1 to 3, wherein the light absorption film and/or heat insulation film and/or heat transfer film are comprised of self-supporting materials.
To achieve the above object, in an optical element according to a fifth invention of this application as defined in any of Claims 1 to 4, wherein a light reflecting film having an aperture large enough for the converged, irradiated control light and signal light to pass, is provided on the control light incidence side of the light absorption film, and is laminated on a heat insulation film and/or a heat transfer film when a heat insulation film and/or a heat transfer film is/are present.
To achieve the above object, in an optical element according to a sixth invention of this application as defined in any of Claims 1 to 5, wherein the light absorption film contains a pigment or a dye molecular aggregate which absorbs light in the wavelength band of the control light.
To achieve the above object, in an optical element according to a seventh invention of this application as defined in any of Claims 1 to 6, wherein a light transmitting film is laminated on a light absorption film, heat insulation film or light reflecting film, and a convex lens which functions as a converging means for the control light is laminated on the incidence side of the control light on the light transmitting film.
To achieve the above object, in an optical element according to an eighth invention of this application, wherein a convex lens is formed on a substrate by filling a resin into a planoconvex lens cavity between a lens plate having at least one depression and the substrate, an optical function point is arranged at the focus of the convex lens, and light incident on the optical function point is converged by the convex lens so that the flux density of the light irradiating the optical function point is increased.
To achieve the above object, in an optical element according to a ninth invention of this application as defined in Claim 8, comprising at least a light absorption film wherein intensity modulation and/or light flux density modulation is/are performed using a thermal lens based on a reversible refractive index distribution produced by respectively converging a control light and a signal light of different wavelengths and irradiating them to the light absorption film, the wavelength of the control light is selected from the absorption band of the light absorption film, and at least the control light is brought to a focus in the light absorption film so as to produce a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region.
To achieve the above object, in an optical element according to a tenth invention of this application as defined in claim 8 or 9, the convex lens is formed by heat melting compression of a thermoplastic resin powder filled in a convex lens type cavity between the lens plate and substrate.
To achieve the above object, in a light control method according to an eleventh invention of this application, intensity modulation and/or light flux density modulation is/are performed using a thermal lens based on a reversible refractive index distribution produced by respectively converging a control light and a signal light of different wavelengths and irradiating them to a light absorption film of an optical element according to any of Claims 1 to 6 or Claim 9, wherein the wavelength of the control light is selected from the absorption band of the light absorption film, and at least the control light is brought to a focus in the light absorption film so as to produce a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region.
To achieve the above object, a light control method according to a twelfth invention of this application is characterized in that intensity modulation and/or light flux density modulation is/are performed using a thermal lens based on a reversible refractive index distribution produced by respectively irradiating the control light and signal light as parallel beams to the convex lens of the optical element according to Claim 7 or 9, and at least the control light is brought to a focus in the light absorption film so as to produce a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region.
To achieve the above object, there is provided a light control method according to a thirteenth invention of this application as defined in Claim 11 or 12, wherein signal light flux in a region strongly affected by intensity modulation and/or light flux density modulation is separately extracted by extracting a signal light flux which diverges after it has passed through the optical element within an angular range smaller than the divergence angle of the signal light flux.
To achieve the above object, in a light control device according to a fourteenth invention of this application, intensity modulation and/or light flux density modulation are performed using a thermal lens based on a reversible refractive index distribution produced by respectively converging a control light and a signal light of different wavelengths and irradiating them to a light absorption film of an optical element according to any of Claims 1 to 6, the wavelength of the control light being selected from the absorption band of the light absorption film, so as to produce a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region, wherein a converging means is provided for respectively converging the control light and signal light, the optical paths of the control light and signal light being so arranged that the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light overlap, and the light absorption film of the optical element is arranged in a position where the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light overlap.
To achieve the above object, in a light control device according to a fifteenth invention of this application, intensity modulation and/or light flux density modulation is/are performed using a thermal lens based on a reversible refractive index distribution produced by respectively converging a control light and a signal light as parallel beams respectively and irradiating them to a convex lens of an optical element according to Claim 7 or 9, producing a temperature rise in the region of the light absorption film which absorbed the control light and the surrounding region, wherein a converging means is provided for respectively converging the control light and signal light, the optical paths of the control light and signal light being so arranged that the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light overlap, and the light absorption film of the optical element is arranged at a position where the regions in which the photon densities are highest in the vicinity of the foci of the control light and signal light overlap.
To achieve the above object, in a light control device according to a sixteenth invention of this application as defined in Claim 14 or 15, the means which separately extracts signal light flux in a region strongly affected by intensity modulation and/or light flux density modulation is a means which extracts a signal light flux which diverges after it has passed through the optical element within an angular range smaller than the divergence angle of the signal light flux.
To achieve the above object, in a light control method according to a seventeenth invention of this application, in the light control method for forming a convex lens by filling a resin in a flat lens cavity between a lens plate having at least one depression and a substrate, said lens plate used is manufactured by a method wherein a photoresist is coated on a base material surface of the lens plate, etching of the base material is performed after forming a pattern having plural apertures on the photoresist, and the diameter of the apertures is increased as etching proceeds by etching the photoresist itself so as to form depressions in the base material surface.